1. Field of the Invention
The present invention relates to an aqueous pigment dispersion which can impart light-shielding properties to a paper base material for wrapping photosensitive materials, foods, phamaceuticals, and the like by coating or printing the dispersion onto the base material and which provides a light-shielding paper which facilitates recycling of waste paper.
2. Description of the Related Art
As a paper base material having light-shielding properties for wrapping a photosensitive material and other materials to be protected from the action of light, which may degenerate due to the action of light, there conventionally exist two typical processes: one is the process in which an aluminum foil and polyolefin resin film are laminated on a base paper; and the other is the process in which polyolefin resin containing carbon black used as a light-shielding pigment is laminated on a base paper or polyolefin resin is laminated on a black base paper into which carbon black is incorporated. However, when these materials are used, maceration properties and separating properties of paper fibers and pigments are extremely deteriorated (at the time of recycling of waste paper), recycling of waste paper is difficult, and these paper materials must be incinerated or buried under the ground as industrial wastes.
As a paper material in which the above-described drawbacks are improved and light-shielding properties and separation properties are both excellent, and a process for manufacturing the paper material, there have been proposed the following: a process in which a mixture of acrylic resin emulsion and wax-type emulsion is applied to a base paper into which inorganic pigments such as carbon black is incorporated or to a base paper surface on which a coating layer of inorganic pigments such as carbon black or a deposit layer such as aluminum is provided (Japanese Patent Application Laid-Open (JP-A) No. 6-184987); a process in which a light-shielding layer containing, as main components, inorganic pigments such as carbon black, aluminum paste, or the like, and a moisture barrier layer containing acrylic resin emulsion and wax-type emulsion are provided on a paper base material (Japanese Patent Application Laid-Open (JP-A) No. 7-82691), and the like. However, although the paper base materials having light-shielding properties, manufactured by these processes, each exhibit maceration properties in which base paper is easily macerated into fibers, these materials do not exhibit sufficient recycling properties in a case that these materials are collected as waste paper to be recycled. For this reason, these materials are impractical for recycling purposes.
When carbon black is used, carbon black cannot be removed unless a deinking process is used. For this reason, in most of common corrugated fiberboard manuracturers in which the deinking process cannot be effected, carbon black appears on a surface of a recycled paper and the commercial value of the recycled paper deteriorates. As a result, practical recycling of such papers cannot be achieved. Further, when aluminum paste is used, aluminum powder cannot be removed even in the deinking process and contamination with the aluminum powder cannot be prevented. For this reason, in this case as well, recycling cannot be achieved.
As a conventional deinking process of waste paper, a flotation process in which ink particles are allowed to adhere to air bubbles, and are floated to be separated, a washing process to be effected by using an appropriate filtering/washing machine and using a combination of a surface active agent, and a decker machine or an inclined screen so that ink particles are stabilized in a washable state, and a combination process in which the flotation process and the washing process are used together are known. When the flotation process is used, high yield is obtained, and an amount of water consumed is less than or equal to a quarter of that in the case of the washing process due to circulating used of recycled water. As a result, the flotation process is superior to the washing method, but the paper obtained by using the flotation process is inferior in quality to that obtained by using the washing process. On the other hand, the washing process generally requires a washing-water purifying equipment, and also requires chemicals. As a result, the washing process is inferior to the flotation process in cost.
Further, since the surface of a paper with carbon black used is black, printing or character typing cannot be rendered thereon. In order to improve printing suitability, whitening processing must be effected for or a printable paper must be laminated onto the surface of the paper.
In addition, as a paper material in which the above-described drawbacks are improved and light-shielding properties, moisture-proof properties, and recycling suitability are excellent, and the process for manufacturing the paper material, there has been known a process in which a light-shielding layer containing, as principal components, yellow pigments such as benzidine yellow, and a moisture-proof layer containing wax-type emulsion and synthetic-rubber-type latex or a resin-type latex are provided on a paper base material (Japanese Patent Application Laid-Open (JP-A) No. 7-1676). The light-shielding properties obtained by this process is limited to light ranging from 250 to 500 nm which is the range of a photosensitive wavelength of a diazo-type photosensitive material required as a wrapping material of a PS (presensitized) printing plate, and do not allow shading of light whose wavelength is in a wide range from ultraviolet region to infrared region, the light being shaded by aluminum foil or carbon black.